1. Field of the Invention
The present invention relates to a back lighting source module for a liquid crystal display (LCD) that is suitable to be used in LCD monitors and LCD televisions. The back lighting source module includes a circuit that is simple and can be modified easily.
2. Description of the Related Art
A liquid crystal display cannot emit light on its own; namely, a back lighting source is required to illuminate the whole panel of the LCD. FIGS. 3A and 3B illustrate a conventional back lighting supply circuit for an LCD. The circuit includes a full bridge converter that uses a pulse width modulation (PWM) controller 90 to drive two sets of transistors 91 and 92 (four power transistors in total) for activating two transformers 93 and 94. Secondary sides of the transformers 93 and 94 are connected in series with each other, and two lighting elements 95 and 96 are connected with the transformers 93 and 94 in series. A feedback circuit 97 formed by several comparators or operational amplifiers feeds current signals of the lighting elements 95 and 96 back to the PWM controller 90. Thus, by means of continuous feedback control, the lighting elements 95 and 96 are provided with stable power supply and emit light uniformly.
In the above-mentioned LCD back lighting module, it is basically formed by two sets of independent power supplies in order to supply power to the two lighting elements; namely, the uppermost dual transistors 91 and the transformer 93, and the lower dual transistors 92 and the transformer 94. They share the PWM controller 90 and the feedback circuit 97 at best. It merely combines two sets of power supplies together without proceeding with an optimal arrangement of the overall circuit. In addition, the above circuit uses four power transistors, which causes high cost, occupies a larger space, and has a complicated structure.
FIGS. 4A and 4B illustrate another conventional back lighting supply circuit for an LCD. The circuit is a two-stage structure formed by a Buck converter 98 and a Royer converter 99, which has a low efficiency and high cost and occupies a larger space.
It is the primary object of the present invention to provide a back lighting source module for an LCD, wherein the module includes only two power transistors for forming a back lighting supply for one ore more lighting elements.
It is another object of the present invention to provide a back lighting source module including a plurality of lighting elements that are connected in series or parallel.
It is a further object of the present invention to provide a back lighting source module that may include a current-limiting circuit when required, thereby balancing the currents in each of the lighting elements such that each lighting element has uniform brightness.
It is still another object of the present invention to provide a back lighting source module including a frequency-variable starting circuit to provide a higher voltage during starting. The voltage returns to a normal value after the lighting elements have been turned on. Thus, the lighting elements are turned on more quickly.
Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.